More particularly, but not exclusively, the protective sheath is intended in its preferred application for protecting fluid ducts, e.g. high temperature air-conditioning ducts. However, it is obvious that a sheath made in accordance with the invention can be installed around any other type of duct such as ducting for water or for compressed air, or even around electrical cables and/or optical cables, assuming that it is thought necessary to protect them.
In said preferred application, and in conventional manner, ducts are initially surrounded by a sleeve, generally made of glass wool lagging that provides thermal insulation for the duct. The sleeve is then surrounded by said sheath which is made of laminated material for the purpose of protecting the duct from the outside and, in the event of a leak from the duct, for the purpose of preventing the fluid that flows at high temperatures escaping to the outside. The sheath is then normally made of glass or synthetic fibers impregnated with a suitable resin. Thus, to improve the characteristics of such protective sheaths, and in particular the stiffness thereof, it is becoming more and more common to use carbon fibers.
Such conventional sheaths are made by means of a cylindrical draping mandrel whose outside geometrical shape, e.g. rectilinear shape, curved shape, or bent shape, corresponds to the shape of the sheath to be obtained, i.e. to the shape of the duct to be covered while making allowance, in the above-mentioned application, for the insulating sleeve. To do this, a sheet of laminated material (fiber-resin) is put into place around the mandrel with one of the longitudinal margins of the sheet extending parallel to the mandrel overlapping its other longitudinal margin such that the two margins overlap over a given angular arc. The sheet wound in this way against the outside surface of the mandrel is then pressed and subsequently hardened by polymerization in order to obtain the protective sheath. Thereafter the sheath is withdrawn from the mandrel and is put into place around the duct to be protected coated with its insulating sleeve, by moving apart the two longitudinal margins of the sheath taking advantage of its intrinsic resilience so as to enable the sheath to pass around the sleeve, after which the longitudinal margins return spontaneously to their initial position where they overlap.
Although removing protective sheaths from said mandrels and, a fortiori, installing them on ducts takes place without difficulty providing the outside geometrical shapes of the ducts to be protected are rectilinear or only slightly curved, when said ducts are bent or have a sinuous outside shapes, removing the resulting sheaths from said mandrels can be difficult because of the separation that needs to be imparted to the longitudinal margins thereof, since this can give rise to major deformation of the sheath and may even exceed its elastic limit.
In addition, removing sheaths when made of carbon fibers is made even more difficult because of the greater stiffness of such sheaths which are then less resilient and therefore more fragile. Consequently, a high proportion of sheaths need to be rejected on being removed from the mandrels because cracking frequently arises that may even amount to breakage of the sheaths, and this is expensive.
It thus appears that the manufacture of protective sheaths made of fibers, and in particular carbon fibers, together with resin is particularly difficult if not impossible when the outside geometrical shape of a duct is complex.
Furthermore, locating a possible leak of fluid that may appear in a duct can be difficult because the sealing of the protective sheath obtained by resilient contact between its two longitudinal margins is poor. Under such circumstances, the fluid passes through the insulating sleeve and spreads all around the protective sheath, escaping via its longitudinal margins.
Furthermore, in the specific application to a duct surrounded by an insulating sleeve of glass wool, it often happens that when the protective sheath is put into place, the sharp edges of the longitudinal margins damage the sleeve.
An object of the present invention is to remedy these drawbacks and to provide a method of manufacturing a protective sheath whereby removal of the sheath from said mandrel and subsequent mounting thereof around a duct can be performed without difficulty and independently of the geometrical shape of the fluid-flow duct.